Electrophysiological signals from human or animal bodies are some of the most fundamental signals used in medical diagnostics. Such signals originate from the muscular, cardiac or neurological electronic activity of a living body.
For recording of heart, muscular or neurological electric activities (including the methods of Electrocardiography (ECG), Electromyography (EMG), Electroencephalography (EEG) and Electrooculography (EOG)) skin-electrodes are normally glued to the skin of the patient. The measured signal is based on the potential between the electrodes, which is dependent on the sum of the neural and muscular electronic activity between the electrodes. The quality of the signal is greatly affected by the accuracy of the position of the electrode and the conductance of the skin. For this reason the surface-skin must be scrubbed off and various fluids or gels are used, to get the electrode in direct galvanic contact with the internal body fluids. This makes the use of electrodes semi-invasive and makes it difficult for other than health-professionals to perform the setup. This can often be a problem, e.g. for sleep research and diagnosis, where overnight measurements are needed.
Sleep Studies
To get accurate results from a sleep study, the patient must feel comfortable and sleep normally. Studies have shown that there is a significant difference between the results from the first night measured and the following nights, when the patient is more comfortable with the studies. Optimally the patient should therefore be measured for two or more nights.
When a full sleep diagnostic study (polysomnography, PSG) is performed, a combination of parameters are measured, including the above mentioned electrophysiological parameters along with large number of other sensor signals. The complication of the setup is therefore high and the setup is fragile and uncomfortable for the patient. The result is that this kind of study is mostly done at a hospital or specialized sleep clinics and for one night only. PSG ambulatory sleep studies performed at people's homes are less common due to these complications; even with the obvious benefits of measuring the patient in his conventional environment and resulting reduction of cost.
Electrocardiography (ECG) is an important tool for sleep diagnostics and gives valuable indicators due to its connection with sleep-related parameters, blood pressure and arousals. Heart-rate-variability (HRV) and pulse-transit-time (PTT) are examples of useful parameters that provide significant indications on the sleep/wake pattern of a subject.
If setup of more complicated sensors, like ECG electrodes, could be performed by a patient or assistant at home, this would increase the quality of the studies, save work and make multiple-night sleep recordings possible.
Capacitive Electrodes
The general idea of capacitive electrodes is to use a different way of measuring up the electrophysiological signals, such as for example ECG signals. When using conventional electrodes, the aim is to provide a good signal connection by minimizing the electronic resistance between the electrode lead and the patient body fluids. The idea behind electro-capacitive electrodes is however instead of basing the signal conductivity on resistance, to form a maximum capacitance connection for the same purpose. As the conductivity of capacitance is variable with frequency this does however require that the amplifier input resistance must be extremely high, for the signal in the band-width of interest to be detected.
The simplest form of a capacitor between the body and the electrode lead would be a metallic plate, where the surface has been coated with a thin layer of isolating material. By pressing the plate towards the body a parallel plate capacitor has been formed. Any electronic activity in the body will cause electronic field to be formed over the isolating material of the plate. By measuring the field or the voltage caused by the field, the electronic body signals can be measured the same way as when conventional electrodes are being used, but without being in direct galvanic contact with the body.
Such capacitive electrode can be generally described by equation (1):C=ε*A/d  (1)where ε is a constant, A is the area of the surface of the electrode, d is the effective insulating distance (the distance between the electrode surface and the bodily fluids constituting the inherent “circuit” of the body).
This kind of electrode was first described in the late 60s and patented in 1970. (P. Richardson and A. Lopez, Jr., “Electrocardiographic and Bioelectric Capacitive Electrode,” U.S. Pat. No. 3,500,823, granted 17 Mar. 1970). The capacitive electrode disclosed by Richardson and Lopez comprises a round disk, 1.5 in diameter, and 0.125 in thick, with an insulating coating on the surface facing the skin of a subject. Typical characteristics of such electrode include a resistance of greater than 4 GΩ (Gigaohms) at 50 V and a capacitance of 5000 pF (picofarad) at 30 Hz.
The general problems of such capacitive electrode include that the signal amplifier used must have an impedance value on par with the high impedance of the electrode and preferably substantially higher, so as not to lose too much of the signal potential, before the signal is measured. A second more complicated problem is that the impedance of the electrode is variable, depending on the distance ‘d’, between the electrode surface and the bodily fluids, which distance will change as a result of bodily movements (e.g. breathing). This second problem has been generally addressed by having a very thin insulating layer on the electrode to increase the capacitance, and by strapping the electrode rigidly to the body so as to minimize the fluctuations in the distance d and thus fluctuations in C.